This post is mostly eye candy but it makes clear what is going on in a certain type of game-based learning, specifically, branching scenario learning (see examples here, here, here). They also display pretty clearly the weakness of the branching scenario: students can figure out pretty quickly what's going on and can realize that success can be earned by memorizing the maze rather than learning the content. I remember watching people play branching scenario laser-disk games in the arcade (back in the days when we had both arcades and laser disks). Rather than watching the movie they close their eyes and toggle though the options: left - right - left - left - etc.